The present invention relates to a method and apparatus for measuring the linear movement between first and second substantially planar structures and is useful, for example, in an automatic focusing system of a photographic enlarging or printing machine.
Photographic enlarging and printing machines are available which automatically move a lens for the purpose of focussing an image. Such machines typically include devices which measure the distance from a film transparency to a lens and the distance from the lens to photographic paper. Using these measurements, the position of the lens to achieve proper focus of the image is calculated and the result is then used to control a motor which moves the lens accordingly. In a high quality machine, it is typically desirable to position the lens extremely precisely, e.g., within 0.0005". To achieve precise positioning, it is desirable to measure the actual position of the lens at its optical center. However, since an image is being projected through the lens, it is not convenient to physically locate a measuring device at the lens optical center since it would interfere with the projected image. It is therefore typical to indirectly measure the lens position as, for example, by measuring the position of some point on the lens support structure intended to move with the lens. For example, some prior art systems measure the position of a motor shaft coupled to the lens support. However, backlash, mechanical play, and distortion of components due to normal operating stresses often introduce errors. In an attempt to minimize such errors, it is common practice to use very precise drive components, excessively heavy structural members, and burdensome assembly and maintenance procedures. All of these factors increase costs and diminish reliability.
Typical prior art systems are disclosed in U.S. Pat. Nos.: 3,832,058; 4,806,988; and Re. 32,424.